1. Field of the Invention
This invention relates to the detection of conditions which would lead to inadvertent criticality in pressurized water reactor (PWR) and boiling water reactor (BWR) nuclear fueled electric power generating units and to providing automatic response, and/or an alarm to which the operator can respond, to terminate the event. It has particular application to the detection of inadvertent boron dilution in a shutdown PWR.
2. Description of the Prior Art
It is imperative that a nuclear reactor which has been shutdown remain subcritical and not inadvertently return to power. Such an event could occur for instance through failure of a component in the complex control system or inadvertent action taken by the operator. In any case, during such an event the reactor approaches criticality exponentially with respect to time thus making it more difficult for the operator to detect the event and take appropriate action before the reactor. The situation is further compounded by a relatively small shutdown margin (the percentage of the reactivity below critical) that can exist when the reactor is shutdown. The status of the core shutdown margin is monitored by monitoring the neutron flux, however, at low neutron count rates the neutron flux count rate becomes a random event with frequent bursts of neutrons occurring between periods of little or no activity. Considering also the fact that the signal level of the neutron flux detector for low count rates is low compared to the considerable noise signal present, and further that the processed signal from the detector is a logarithmic and not a linear function of the neutron count rate, it can be appreciated that it is very difficult to detect an inadvertent approach to criticality early in the event and at the same time avoid spurious alarms.
Both BWR and PWR units utilize control rods which are inserted into and withdrawn from the reactor core to regulate reactor reactivity through controlled absorption of the neutrons released by the fission process. In addition, PWR units employ boron, also a neutron absorber, dissolved in the reactor coolant water, to regulate reactivity in conjunction with rod control. In fact, long duration changes in reactivity are normally established through the boron control system. When the reactor is to go to power, the boron concentration in the reactor coolant water is diluted at a controlled rate. Similarly, when the reactor is shutdown, boron concentration is increased to establish the shutdown margin. During refueling operations, the reactor vessel is filled with heavily borated water while spent fuel assemblies are removed from the core and replaced by fresh fuel assemblies.
The boron control system includes a number of tanks, pipes, valves and controls which make up borated water of regulated concentrations from boric acid and fresh water for regulating reactivity insertions and for use during refueling. The heavily borated water in the refueling water tank is also available for use in the safety injection system should the normal control systems be unable to shutdown the reactor when predetermined limiting operating parameters are approached.
While under normal operating conditions the boron concentration in the reactor coolant system is closely controlled, it is possible that it could become inadvertently diluted such as mentioned above through component failure or inadvertent operator action. If this occurs when the reactor is at power or during startup, currently provided protection system actions such as the high neutron flux reactor trip and the rod insertion limit alarm warn the operator of the condition, and in some cases, initiate corrective action. Inadvertent dilution during refueling is prevented by locking closed the valves through which dilution water would have to flow thereby isolating the reactor coolant system.
None of the present reactor protection systems are suitable for detecting inadvertent boron dilution events, when the reactor is shutdown, in time to prevent the reactor from going critical. While inadvertently going critical due to boron dilution is not totally unacceptable in itself since as the reactor power increases competing effects such as a rise in fuel temperature will cause the reactor to go subcritical and then oscillate between critical and subcritical, the pressure transients generated during these excursions are unacceptable.